1. Field of the Invention
The invention relates to a method for heating a device for delivering a liquid additive.
2. Related Art
Devices for delivering a liquid additive are used for example in the automotive field for supplying a liquid additive to the exhaust-gas treatment device of an internal combustion engine. Exhaust-gas treatment devices are widely used in which the reduction of nitrogen oxide compounds in the exhaust gas takes place with the aid of a liquid additive. Corresponding delivery devices can be used for exhaust-gas treatment devices of this type.
An exhaust-gas purification method particularly widely used in such exhaust-gas treatment devices is the process of selective catalytic reduction [SCR process]. In this process, ammonia is often used as a reducing agent for the reduction of nitro-gen oxide compounds. Ammonia can be obtained in an exhaust-gas treatment device from a urea-water solution. Here, the urea-water solution is converted to ammonia by thermolysis (thermally) and/or by hydrolysis (with the aid of a hydrolysis catalytic converter). The urea-water solution may be supplied to the exhaust-gas treatment device as liquid additive by a described delivery device. A corresponding urea-water solution is available for example under the trade name AdBlue® with a urea content of 32.5%.
A problem for devices for delivering liquid additive is that the additives (such as for example the described urea-water solution) can freeze at low temperatures. The described urea-water solution, for example, freezes at −11° C. In motor vehicles, such low temperatures arise for example during long standstill phases in winter. For this reason, heaters are often provided in devices for the delivery of liquid additives. By the heaters, the frozen additive in the device can be thawed, or the freezing of the additive can be prevented or slowed. Heaters are normally operated with electrical energy. Liquid-type heaters' are also known in which a heating fluid flows through a heating loop. In heating loops, (heated) cooling water of an internal combustion engine is typically used as heating fluid.
A problem with such heaters is that the introduction of heat often does not take place in the direct vicinity of the frozen additive, and therefore special structure is required for conducting the heat from the heater to the additive. Such structure for heat conduction often cannot be integrated, or can be integrated only with difficulty, into a delivery device. Furthermore, it is also the case that, with such structure, the heat generated by the heater can normally be dissipated to the liquid additive only after a time delay and with heat losses. Furthermore, the materials that are typically in contact with the liquid additive in a delivery device often have only low thermal conductivity, and therefore act as heat insulation between a heater and the liquid additive. If urea-water solution is used as liquid additive, the materials are, for example, special urea-resistant plastics. This additionally makes it difficult to provide structure for heat conduction.